1. Field of the Invention
The present invention relates to a liquid discharge head having nozzles for discharging liquid, and a head cartridge and an image forming apparatus that use such a liquid discharge head.
2. Description of the Related Art
Ink discharging methods of the ink jet recording type that have been widely used today may be roughly classified into a method that uses an electro-thermal converter (heater) as a discharge energy generating element that is used for discharging an ink droplet, and a method using a piezoelectric element for the same purpose. In both methods, it is possible to control discharge of an ink droplet using an electrical signal. For example, the principle of the ink droplet discharging method using the electro-thermal converter is that, by giving an electrical signal to an electro-thermal converter, ink near the electro-thermal converter is boiled in an instant and, based on rapid growth of a bubble that is generated due to a phase change of ink thereupon, an ink droplet is discharged at high speed. On the other hand, the principle of the ink droplet discharging method using the piezoelectric element is that, by giving an electrical signal to a piezoelectric element, the piezoelectric element displaces and, based on a pressure generated upon this displacement, an ink droplet is discharged. The former method has merits such that it is not necessary to so much space for the discharge energy generating element, a configuration of an ink jet print head is simple and thus integration of nozzles is easy, and so on.
Recently, following the increasing processing speed of personal computers and the spread of the Internet and so on, the demand for high-speed processing of color images have been increasing more and more, i.e. the demand for quick printing of recorded materials with high fineness and high gradation, that is, so-called extremely high-quality recorded materials, has been increasing, so that printers that can produce high-quality images at high speed have been demanded.
For obtaining a high-quality image with high fineness and high gradation, such a method is suitable that performs recording by discharging a very small ink droplet from each nozzle. On the other hand, for speed-up, it is necessary to discharge ink droplets repeatedly at a short period from nozzles. Further, it is also required that a carriage mounting thereon a print head moves at high speed synchronously with a response frequency of the head. When performing recording by repeatedly discharging a very small ink droplet from each nozzle as described above, a stripe 101, for example, is generated in an image of a paint-out printing portion, i.e. a solid printing portion 100, of a bar graph as shown in FIG. 10. The stripe 101 just corresponds to a transition portion between nth scan and (n+1)th scan.
FIG. 11B is an enlarged view of this transition portion, and FIG. 11A shows the state thereupon wherein ink droplets 102 are discharged from a head 103. When image data is solid, nozzles SEG0 to SEG255 are all driven at a high response frequency. Thus, due to discharge of ink droplets 102 from those nozzles in end regions of the image data, the air with viscosity around the discharged ink droplets moves at a speed substantially equal to that of the ink droplets. Then, the air over the whole discharge port array moves in the same direction as the ink droplets 102 so that a pressure-reduced state is generated in that portion. Accordingly, the air other than the air around the discharged ink droplets moves toward the pressure-reduced portion, so that air flows as shown by arrows in FIG. 11A are generated. As a result, a discharge direction of the ink droplets 102 discharged from the nozzles located in end regions of the nozzle array is dethroughted or mis-aligned toward the center of the nozzle array from expected positions due to the air flows, i.e. inward jetting of the ink droplets occurs. Further, due to air flows generated upon movement of a carriage in a main scanning direction upon recording, air flows toward the center of the nozzle array are generated, so that the discharge direction of the ink droplets 102 discharged from those nozzles located in the end regions of the nozzle array are mis-aligned toward the center of the nozzle array due to the air flows. As a result, there has been a problem that hit positions of the ink droplets on a recording medium are mis-aligned to cause the stripe 101 as shown in FIG. 11B ((dot) mis-alignment). If the discharge amount of ink is increased for preventing the generation of the stripe 101, waviness is caused on the recording medium due to overflow of ink from the recording medium or absorption of ink into the recording medium, thereby deteriorating a recording image. Particularly, in the image formation with high fineness and high resolution, it is important to reduce graininess and reproduce fine lines, and thus, it is required that a dot size be as small as possible. Accordingly, also from this aspect, it is not desirable. On the other hand, if a period for repetitive discharge of ink droplets is prolonged, the generation of air flows is eased. However, the speed of the printer is lowered, so that it is not possible to satisfy the users"" demand for high-speed printing.
Therefore, it is an object of the present invention to provide a liquid discharge head that is capable of high-speed recording and can reduce generation of a stripe in a recording image, and further provide a head cartridge and an image forming apparatus that use such a liquid discharge head.
As a result of detailed review by the inventors, the following has been made clear.
(a) Not only the ink droplets discharged from those discharge ports located at end portions of the discharge port array are mis-aligned. For example, even in the state where ink droplets 102 are discharged from discharge ports of part of a head 103 as shown in FIG. 12A, the ink droplets 102 discharged from the discharge ports located at end portions in the discharge port array direction are mis-aligned toward an intermediate portion of the discharge ports. On the other hand, a stripe 101 between solid portions 100 becomes conspicuous in an image as shown in FIG. 12B when it appears at a transition portion between recording scans. Thus, particularly the end portions of the discharge port array causes a problem.
(b) As the number of discharging discharge ports upon recording is reduced, a mis-alignment amount of liquid droplets discharged from discharge ports at end portions of an image is reduced. It is estimated that this is because, due to the reduction in number of the discharging discharge ports, a pressure-reduced state at the intermediate portion of the discharging discharge ports is weakened so that air flows that cause mis-alignment become reluctant to occur. A relationship between the number of discharged liquid droplets and the mis-alignment is shown in FIG. 6 as a specific example, which will be described later. No mis-alignment in a direction perpendicular to the main scanning direction due to air flows occurs when only one liquid droplet is discharged. As the number of the discharged liquid droplets increases, the mis-alignment amount is increases. It is also estimated that, also in case of reduction in discharged liquid droplet amount, air flows become reluctant to occur inasmuch as a pressure-reduced state-at the intermediate portion of the discharging discharge ports is weakened.
(c) Further, as resolution in a sub-scanning direction for recording is lowered, the mis-alignment amount of liquid droplets discharged from discharge ports at the end portions is reduced.
(d) The present inventors have made detailed review based on the foregoing air flow model, and configured such that a volume of each of discharge ports at end portions is greater than a volume of each of discharge ports at an intermediate portion. As a result, the mis-alignment amount of liquid droplets discharged from the discharge ports at the end portions was reduced. As described above, it is estimated that this is because, due to the reduction in volume of the intermediate portion of the discharge port array, a pressure-reduced state caused by flying of liquid droplets at the intermediate portion was weakened. By incorporating it into a liquid discharge head that performs gradation recording by preparing discharge ports whose discharge amounts of liquid droplets are different, the foregoing misalignment amount can be achieved without increasing the size of the head.
For accomplishing the foregoing object based on the foregoing results of review, according to the present invention, there is provided a liquid discharge head having a plurality of discharge port arrays each having a plurality of discharge ports and each arranged substantially in parallel to a print medium conveying direction, and a plurality of discharge energy generating portions for discharging liquid from the discharge ports, respectively, the liquid discharge head moved to scan in a direction crossing the conveying direction, the liquid discharge head characterized in that each discharge port array includes first discharge ports each for discharging a liquid droplet of a first volume, and second discharge ports each for discharging a liquid droplet of a second volume being smaller than the first volume, the discharge port arrays that are adjacent to each other have each of the first and second discharge ports as a pair in the scanning direction, and at least one of the discharge port arrays is a first discharge port array including first discharge port groups respectively having the first discharge ports arranged on both end sides of the at least one discharge port array, each of the first discharge port groups including an end discharge port of the first discharge ports that discharges liquid contributing to image formation, the first discharge port array further including at least one second discharge port group having the second discharge ports arranged between the first discharge port groups.
In the liquid discharge head of the invention as configured above, the first discharge port groups having the discharge ports each discharging a liquid droplet of a large volume are arranged on both end sides of the discharge port array so as to include end discharge ports that contributes to image formation, and a volume of each of liquid droplets discharged from the discharge ports forming the second discharge port group arranged between the first discharge port groups is set small. With this configuration, a pressure-reduced state caused by flying of the liquid droplets discharged from the intermediate portion of the discharge port array is weakened, so that the mis-alignment amount of the liquid droplets discharged from the discharge ports located at the end portions of the discharge port array can be reduced.
Further, the liquid discharge head of the present invention can achieve reduction of the foregoing mis-alignment of the liquid droplets without increasing the size of the head in the configuration that can perform gradation recording.
It may be configured that a discharge port area of each of the first discharge ports forming the first discharge port group is greater than a discharge port area of each of the second discharge ports forming the second discharge port group.
It may be configured that each of the discharge energy generating portions has an electro-thermal converter that generates thermal energy for causing film boiling in liquid to discharge the liquid from the discharge ports.
According to the present invention, a head cartridge is characterized by comprising the liquid discharge head of the present invention and a liquid tank storing liquid to be supplied to the liquid discharge head.
It may be configured that the liquid tank is detachable relative to the liquid discharge head through attaching/detaching means.
According to the present invention, an image forming apparatus is characterized by comprising a mounting portion for the liquid discharge head of the present invention, wherein an image is formed on a print medium using liquid discharged from discharge ports of the liquid discharge head.
As described above, since the image forming apparatus of the present invention forms an image on a print medium using the liquid discharge head of the present invention, the amount of mis-alignment in which a liquid droplet is not discharged to an expected position on the print medium, which has been the conventional problem, is reduced. Thus, even if solid printing is carried out, a high-quality print image with high fineness and high gradation can be obtained without generating a stripe.
It may be configured that the mounting portion has a carriage that is movable for scanning in a direction crossing a print medium conveying direction.
Further, it may be configured that the liquid discharge head is detachably mounted on the carriage through attaching/detaching means.